This invention relates to optical fiber cables which are suitable for use within building vertical shafts and also are provided with protection against moisture internal migration. In particular, this invention relates to single-tube design optical fiber cables suitable for such uses.
Drop cables are outdoor cables which bring telephone service to buildings, whereas riser cables provide telephone service within buildings. Riser cables extend upwards from basement vaults to wiring closets located on upper floors.
Drop cables must be able to withstand the changing conditions found in the outdoor environment. Most materials increase in length and volume with increases in temperature and decrease in length and volume with decreases in temperature. Each material may have a different rate of change of length given a specified change in temperature. Such a rate is called the coefficient of thermal expansion for a material. Because different materials in a cable may have different coefficients of thermal expansion, temperature changes may induce strains in the cable components. For this reason, changes in optical fiber attenuation over different temperatures are measured in cables intended for outdoor use. Successful cables must not experience unacceptable increases in optical fiber attenuation caused by cable strains induced by temperature-related conditions.
Drop cables also must be protected against migration of moisture within the cable. Although cable jackets are intended to prevent the ingress of water into the cable, no plastic material perfectly stops the ingress of moisture. Furthermore, water may enter a cable at points where the cable jacket has been damaged, or at the end of the cable. Therefore, longitudinal movement of water along the inside of the cable must be prevented. For this reason, water-blocking or water-absorptive material is provided in cable interstices which otherwise could act as conduits for moisture internal migration. Types of materials which may be used for this purpose are gel-like filling and flooding compounds. Filling compounds are disposed alongside the optical fibers within buffer tubes, while flooding compounds are disposed in spaces between the cable jacket and the buffer tubes holding the optical fibers. Many filling and flooding compounds are oil or grease-based. As a result, most filling and flooding compounds provide fuel for combustion. However, most cables intended for outdoor use are not required to be flame-retardant.
Other types of materials are becoming more widely used in outdoor use cables for protection against cable moisture migration. Examples include water-absorptive polymers, which may be inserted into a cable as loose powders or incorporated into tapes which are wrapped about other cable components. Another example is water blocking strength members, as disclosed in U.S. Pat. No. 4,913,517 and 5,389,442.
Cables intended for use within buildings normally are not exposed to the moisture and extreme temperature conditions experienced by cables intended for outdoor use. However, building cables are required by the National Electrical Code to meet criteria indicating that the cables will not act to spread fires within a building. The most well-known test standard for riser-rated cables is Underwriters Laboratories (UL) Standard 1666, xe2x80x9cTest for flame propagation height of electrical and Optical-fiber Cables installed vertically in Shaftsxe2x80x9d (Second Edition, Jan. 22, 1991). The second edition of this standard is referred to herein as UL Standard 1666.
An optical fiber service cable designed to be suitable for both indoor and outdoor use is disclosed in U.S. Pat. No. 5,566,266, which issued on Oct. 15, 1996 in the names of Navxc3xa9 and McDowell. However, the disclosed cable is designed for use with a rather high optical fiber count and discloses an inner tube which itself has an outer diameter of 10.2 mm. Such a cable could not be connectorized using standard buffer tube fanout kits. The cable also employs a tape formed from materials such as a polyimide. Such tapes significantly add to the cost of the cable, and it is necessary to process and splice such tapes.
It is therefore an object of the present invention to provide riser-rated cables having a small diameter and low minimum bend radius which also is formed using low-cost materials.
These and other objects are provided, according to the present invention, by fiber optic cables suitable for both outdoor and indoor applications, comprising: a core tube having an OD of about 3.0 mm or more surrounding a plurality of coated optical fibers; a jacket formed of UV-resistant flame-retardant polymer material surrounding said core tube; and at least one layer of strength members disposed between said core tube and said jacket. The jacket can have an OD of about seven mm or more. The coated optical fibers can experience a short-term increase in signal attenuation of no more than about 0.01 dB when the cable is looped in a radius of 5 centimeters. The strength members can be wrapped around the core tube in opposite directions and the set of two strength member layers can be disposed between and directly contiguous to said core tube and said jacket. The cable is capable of meeting the flame retardance requirements set out in UL Standard 1666 in the absence of a flame-resistant tape. The strength members may be impregnated with a water blocking material.